\chapter{APPARATUS}
\label{ch:app}
 
The measurements described in this thesis were performed
with the SLAC Large Detector (SLD) operating at the 
SLAC Linear Collider (SLC).    The SLC is capable of 
colliding polarized or unpolarized electrons with
unpolarized positrons at a center of mass energy at or
near the \z0 resonance.
 
The SLC was conceived as an upgrade to the existing linear
accelerator at SLAC.  The first \z0 boson was produced by
the collider in the Spring of 1989 and measured by the 
Mark II detector.  The SLD detector was designed and
constructed specifically for \z0 physics at the SLC.
The SLD collaboration consists of approximately
150 active physicists from 34 institutions.  (A complete
list of SLD collaborators and institutions may be
found in Appendix~\ref{ch:appsld}.)  The SLD
was first moved onto the SLC beamline
in February of 1991 and underwent an ``engineering run'' from
June through August of that year.
 
The 1992 SLC run began with an unpolarized electron beam.
Roughly 1000 hadronic \z0 decays were measured with the
SLD detector.  In April of 1992, a polarized electron 
source was installed and commissioned.  
From May through August, SLC produced approximately 10,000
\z0 events with an average electron beam polarization of
23\%.  Both luminosity and polarization were improved for
the 1993 run.  SLD logged approximately 50,000 \z0  events 
to tape
with an electron beam polarization of 63\% from March 
through August of 1993.
 
 
% other slc layout
%\begin{figure}[tbp]
%  \vspace*{0.9in}
%  \hspace*{3.5in}
%  \epsfysize=7in
%  \epsffile[129.6 -9 352.8 446.4]
%{/net/bovine/home/kevin/ps/the/ch3/slc_layout.ps}
%  \caption{The SLAC Linear Collider}
%  \label{fig:slc}
%\end{figure}
 
 
\section{The SLAC Linear Collider}
 
 
 
The SLC is the first ever linear collider \cite{slcdr}
and shown schematically in Figure~\ref{fig:slc}.
Linear colliders are unique in that electrons
and positrons must be accumulated, accelerated and 
brought into collision on each machine cycle.  While
%the accelerator physics in
this environment offers a set of challenges very different
from that of storage rings, it also offers unique 
opportunities in both electroweak and heavy flavor physics. 
The SLC parameters are given in Table~\ref{tab:slc}.  These
numbers, along with an uptime of 65-75\%, allowed SLC to
deliver over 50,000 \z0s to SLD with high polarization in 1993.
 
 
\begin{figure}[htbp]
%  \vspace*{0.9in}
  \hspace*{0.5in}
  \epsfxsize=5.in
  \epsffile[0. 201. 531. 523.]
{/net/bovine/home/kevin/ps/the/ch3/slc.eps}
  \caption{The SLAC Linear Collider (SLC).}
  \label{fig:slc}
\end{figure}
 
\begin{table}[bht]
\renewcommand{\arraystretch}{1.3}
\caption[Table of SLC Parameters.]{Table of SLC parameters.}
\begin{center}
\begin{tabular}{||c||c|c|c||} \hline
parameter & symbol & design goal & achieved\footnotemark \\ 
\hline \hline
frequency & f & 180 Hz & 120 Hz \\ \hline
beam spot size & $\sigma_x,\sigma_y$ & 2$\mu m\,\times2\,\mu m$ & $2.2 
\mu m\,
\times\,
1.7 \mu m$ (1992) \\ 
at the IP & & & 2.6 $\mu m\,\times\, 0.8 \mu m$ (1993) \\ \hline
bunch intensity & $N_{e^+},N_{e^-}$ & $5\times10^{10}, 5\times10^{10}$ &
$ 3\times10^{10}, 3\times 10^{10}$ \\ \hline
polarization & $ {\cal P}_e $ & $50\% $ & $63 \% $ \\ \hline
luminosity & ${\cal L}$ & $10^{30} /cm^2/sec $ & $3.8\times 10^{29}/cm^2/sec$ 
\\
& & & $\simeq 40 Z^\circ/hr$ \\ \hline \hline
\end{tabular}
\end{center}
\label{tab:slc}
\renewcommand{\arraystretch}{1.0}
\end{table}
The following sections will describe the main aspects
of the SLC, with particular attention given to the
aspects which involve the creation and transport of a
polarized electron beam.
 
\subsection{Polarized Electron Source}
\label{se:pes}
 
A longitudinally polarized electron beam
may be created by irradiating a GaAs semiconductor
cathode with a circularly polarized laser \cite{polprop}.
The energy of the photons must be slightly greater
than the band gap energy for photoemission to take place.
Angular momentum conservation is responsible for the 
electron polarization, as  
P$_{{3\over{2}}}$ valence band electrons are promoted
to the S$_{{1\over{2}}}$ conduction band by the circularly polarized
photons in a 3:1 ratio.  Therefore, the 
maximum polarization possible in
conventional bulk GaAs cathodes is  
$\Pe = (3-1)/(3+1) = 50\% $.  The true polarization
is always below this limit due to excitation of 
conduction band electrons.
 
 
 
\begin{figure}[tbp]
  \hspace*{1.2in}
  \epsfysize=3.5in
  \epsffile[9.4 128. 433. 570.]
{/net/bovine/home/kevin/ps/the/ch3/gascath_pol.ps}
  \caption[Electron Polarization as a Function of Source
Laser Wavelength.]
{Electron polarization as a function of source
laser wavelength ($\lambda$).  SLC utilized a bulk GaAs 
cathode in 1992 and a strained GaAs cathode in 1993.}
  \label{fig:polvlamda}
\end{figure}
 
 
More recently, strained GaAs cathodes have been shown
to produce polarizations well over 50\% \cite{strained}. 
The 
strained lattice removes the degeneracy of the 
$m_j = 3/2$ and $m_j = 1/2$ levels of the P$_{3\over2}$ valence state 
and theoretically allows for 100\% polarization.
 
 
Figure~\ref{fig:polvlamda} shows the electron polarization
for different types of cathodes as a function of laser wavelength.
In 1992, SLC ran with a bulk GaAs cathode and a laser
wavelength of 715 $\!\rm{nm}$.   This produced an electron
beam with $\sim\!28$\% polarization.   In 1993, the
cathode used was a strained GaAs wafer.  For the first
portion of the 1993 run, the laser wavelength was 850 $\!\rm{nm}$,
producing a polarization of $\sim\! 55$- $\!60\%$.    
For the remaining 80\% of the 
1993 run, the laser wavelength was 865 $\!\rm{nm}$, yielding an electron
beam polarization of $\sim\! 62$- $\!67\%$.
 
 
 
The 
photo-emitted electrons are accumulated into a ``bunch''
by a 178 MHz RF field.  The electrons are then accelerated through a 
high-gradient field to an energy of 50 MeV.  Figure~\ref{fig:pes} shows the layout of the
polarized electron gun and laser. The laser strikes
the cathode two times per 
%1/120th of a second 
120 Hz machine cycle.  One
bunch of electrons will eventually comes into collision with
positrons at the interaction point, while the other electron bunch is
used to create positrons.  Typically, between $6$ and $10\times 10^{10}$
electrons are liberated from the cathode per pulse.
 
After reaching an energy of 50 MeV, the electrons enter the
first section of the linear accelerator (LINAC)
where they, along with one
positron bunch from the positron target, are accelerated to
an energy of 1.19 GeV.  The electrons and positrons are then
diverted from the LINAC into two different damping
rings.
 
 
 
 
\begin{figure}[hbt]
  \hspace*{0.9in}
  \epsfysize=3.5in
  \epsffile[216 295.1 496.8 496.8]
{/net/bovine/home/kevin/ps/the/ch3/pes.ps}
  \caption{The SLC Polarized Electron Source.}
  \label{fig:pes}
\end{figure}
 
 
 
\footnotetext{The parameters listed in this column were achieved
during or before the 1993 SLC run.   Improvements on bunch
intensity and spot sizes are planned for subsequent SLC runs.}
 
 
 
\subsection{Damping Rings}
 
The damping rings serve to ``cool'' the beams so that
they may be accelerated.  The smaller the spread in both momentum and
position space allows for fewer losses during acceleration as well
as lower backgrounds in the detector.  Both electron and positron beams
are stored at 1.19 GeV.  Since the
positrons  have a larger energy spread as they are captured
from the positron target (see Section \ref{se:pprod}), they
must be damped for two machine cycles ($\sim\!16.6\,\rm ms$),
while the electrons are damped for only 1 cycle 
($\sim\!8.3\,\rm ms$).
%The electron damping ring is shown in Figure~\ref{fig:dr}.
 
%\begin{figure}[tbp]
%  \hspace*{0.45in}
%  \epsfysize=4.5in
%  \epsffile[-14.4 -57.6 756 633.6]
%{/net/bovine/home/kevin/ps/the/ch3/damping_ring.ps}
%  \caption{The electron damping ring.}  Note that the
%linac-to-ring (LTR) solenoid is used to store the spins.
%  \label{fig:dr}
%\end{figure}
 
 
 
In order to preserve polarization in the damping ring, the
electron spin vector must be pointing parallel to the magnetic
field direction of the bending magnets. 
This is achieved by passing the beam
through a solenoid field as it enters the damping ring.
The spin vector is flipped from along the direction of
motion to perpendicular to the plane of the damping ring.  
 
If the SLC is running round beams 
($\epsilon_x\!\simeq\! \epsilon_y$) the beams 
have their spin vector oriented in the horizontal plane
while being accelerated.   Spin procession in the arcs
brings the electron spin longitudinal at the interaction
point.  
This means that another
solenoid, the ring-to-linac solenoid (RTL)
must be used to flip the spins back to longitudinal
after the electrons exit the damping ring.
When SLC is running in ``flat'' beam mode
($\epsilon_x\! \simeq \! 10\epsilon_y$), the RTL solenoid
disrupts the beam by coupling the horizontal and vertical
emittance (the flat beam passing through the solenoidal 
field gets rotated in the transverse plane) and must be turned off.  
The electrons are accelerated with their spins pointing up
and the spin is made longitudinal
through a series of ``spin bumps'' in the arcs (see Section~\ref{se:arcs}).
 
 
The SLC was designed to run 
with round beams and did so for the entire 1992 run.
Late in the 1992 run, some tests were performed with
the SLC in flat beam mode for the Final Focus Test
Beam Facility, which performs research and development
for a future high energy linear collider.  Flat
beam acceleration and transport down the LINAC was
so successful that it was decided to bring the beams
through the arcs and into collision.  Record specific
luminosities were set almost immediately and it was
determined that transport of flat beams through the
arcs was not as difficult as once anticipated.   The smaller
spot size, as well as lower emittance in the vertical plane
allowed for higher luminosity with lower backgrounds in the
SLD. Consequently,
the SLC ran in flat beam mode for the bulk of the 1993 
run and plans to continue
to do so in future runs \cite{flatbeams}.
 
 
\subsection{Positron Production}
\label{se:pprod}
 
Three bunches of particles are accelerated during each machine
cycle.   The
first two bunches are the positrons and electrons (respectively)
which are to be brought into collision.  The third bunch is
known as the ``scavenger'' electron bunch.  This bunch is diverted
from the linear accelerator at approximately 2/3 the way down its length
and
steered into
a tungsten positron production target.  The resulting electromagnetic
showers produce both electrons and positrons.  Positrons in the
energy range 2-20 MeV are 
captured and returned to the front end of the LINAC where they are
accelerated and stored in the  damping ring. 
Typical yields from the positron
target are one positron per incident electron.
 
\subsection{Linear Accelerator}
 
The bunches are accelerated down approximately 3 kilometers
of
linear accelerator (LINAC).  The beams  gain energy from
radiofrequency cavities which are pumped with energy from
klystrons.  The accelerating gradient is 17 MeV/m.
In typical running conditions, $\sim\!3\times 10^{10}$
particles per bunch are accelerated down the linear 
accelerator.  
 
\subsection{Arcs}
\label{se:arcs}
 
After being accelerated to $\sim\!0.9$ GeV above collision
energy in the linac, the electrons and positrons are steered through 
two opposing arcs of $1\,$km in length.  The beams ``coast''
through the arcs, losing energy by synchrotron radiation.  
The arcs do not lie in the horizontal plane, 
so the beam transport
is complicated by motion in both dimensions perpendicular
to the momentum of the electrons.
 
In flat beam mode, the electron
arc is utilized to flip the spin vector from transverse
to longitudinal by way of ``spin bumps'' \cite{spinbumps},
which utilize a strong resonance between the vertical 
betatron tune and the spin tune.  Spin bumps are vertical
orbit distortions which a separated by a fixed spin rotation
around the vertical axis.  The net effect of the spin bumps
in the arc is to bring the electron spin from transverse entering
the arcs to
longitudinal exiting the arcs (entering the final focus).
 
\subsection{Final Focus}
 
To make up for the relatively low repetition rate of a
linear collider, the beams must be focused very strongly
at the interaction point.  This is achieved at the SLC
via three superconducting quadrupole magnets.  The
final quadrupole is approximately 1.5 meters from the
interaction point (IP).  At the point of collision, the
dimensions of the beams (in flat-beam mode) are $\sim\!3\,\mu$m in the
horizontal, $\sim\!0.8\,\mu$m in the vertical and
$\sim\!1\,\mm$ along the beam axis.  The size and location
of the IP is very stable over time (see Section~\ref{se:beampar}).
 
\section{The SLAC Large Detector}
 
Collisions are measured by a single multipurpose
detector, the SLAC Large Detector \cite{slddr}.  The SLD, shown in Figure~\ref{fig:sldquad},
combines excellent tracking,
calorimetry and particle identification into a state-of-the-art 
high energy physics apparatus.
 
\begin{figure}[phbt]
  \hspace*{-0.3in}
%  \vspace*{-.3in}
  \epsfxsize=7.in
  \epsffile[22. 233. 700. 853.]
{/net/bovine/home/kevin/ps/the/ch3/sldcut.eps}
  \caption[Cutaway View of the SLD.]{Cutaway view of the SLAC
Large Detector.}
  \label{fig:sldcut}
\end{figure}
 
 
\begin{figure}[hbt]
%  \vspace*{.4in}
  \hspace*{0.7in}
  \epsfysize=4.9in
%  \epsffile[-316.8 -216 100.8 259.2]
  \epsffile[-316.8 -180 100.8 295.2]
{/net/bovine/home/kevin/ps/the/ch3/sld_quadrant.ps}
  \caption[Quadrant View of the SLD.]{Quadrant view of the SLD.}
  \label{fig:sldquad}
\end{figure}
 
The SLD is a cylindrically symmetric detector with
a 0.6 Tesla solenoidal magnetic field to momentum analyze
charge particles.  The detector has a central ``barrel''
system that surrounds the interaction point and two
``endcaps'' which cover the forward angles.  During down
times, the endcaps may be withdrawn for access to the
interior components of the detector.
 
The right-handed
coordinate system used by the SLD is a normal spherical
polar system:
\begin{itemize}
\item the z axis is defined by the positron beam (pointing North)
\item azimuthal angle ($\phi$) runs from 0 to 2$\pi$, the x-axis ($\phi$=0)
points to the West.
\item polar angle ($\theta$) runs from 0 (North) to $\pi$ (South).
\end{itemize} 
 
The analyses discussed in this thesis make use of the
SLD calorimetry systems: the liquid argon calorimeter and
the luminosity monitor.  The following sections will
describe all of the major subsystems of the SLD.  However, calorimetry
will be emphasized.
 
 
 
\subsection{Tracking}
 
SLD possesses three major tracking systems, the Vertex
Detector, the Drift Chambers and the Warm Iron Calorimeter
(WIC) Strips.
 
\subsubsection{Vertex Detector}
 
Taking advantage of the small beampipe at the interaction
point and the low repetition rate of the SLC, the
SLD Vertex Detector utilizes charge coupled devices
to make high resolution space point measurements of 
charged particle tracks.  When track points are linked
with tracks reconstructed by the Central Drift
Chamber, secondary vertices from heavy quark and tau
lepton decays can be resolved with high precision.
 
\begin{figure}[hbt]
  \hspace*{.5in}
  \epsfxsize=5.5in
%  \epsfysize=4.9in
  \epsffile[0. 112. 596. 607.]
{/net/bovine/home/kevin/ps/the/ch3/vertex.eps}
  \caption{The SLD Vertex Detector.}
  \label{fig:vxd}
\end{figure}
 
 
The Vertex Detector (shown in Figure~\ref{fig:vxd}) 
is comprised of 480 CCDs which surround
the interaction point in four concentric barrels. 
Each CCD contains approximately 400$\times$600 pixels each of
size 22$\times$22 microns.  The data from 120 Mpixels is read
out and condensed to approximately 50-80 Kbytes per
event \cite{vxd}.
 
\subsubsection{Drift Chambers}
 
Before linking tracks with hits in the Vertex Detector,
tracks are found and fitted with the Central Drift Chamber
(CDC) and the Endcap Drift Chambers (EDC).   The drift 
chambers are gas-wire tracking systems.  The CDC 
contains a cylindrical arrangement of wires which run
approximately parallel to the beam line.  Ionization
from charged particles passing through the chamber drifts
to the wires in the presence of large electrostatic fields.
The wires are instrumented on both ends, so the z component
of the tracks may be found via charge division.
 
The Endcap Drift Chambers have wires running perpendicular
to the beamline for track reconstruction at smaller angles.
The chambers are divided into an inner and outer chamber
which are separated by the Endcap CRID (see Section~\ref{se:pid}).
Backgrounds from the SLC as well as material in front of
the EDCs have thus far hampered their ability to find charged tracks.
 
 
\subsection{Particle Identification}
\label{se:pid}
 
When the velocity of a particle exceeds
the speed of light in a medium, the
particle emits \v Cerenkov radiation.
The \v Cerenkov angle is related to the
velocity of the particle.  From the measurement of the
\v Cerenkov angle, coupled with
momentum information from the drift 
chamber, the mass of the particle and
hence the identity of the particle can
be ascertained.  The
\v Cerenkov Ring Imaging Detector (CRID)
is designed to measure
the \v Cerenkov angle of tracks and
therefore perform particle identification \cite{crid}.
 
\begin{figure}[hbt]
  \hspace*{-.2in}
  \epsfxsize=5.5in
  \epsffile[19. 0. 600. 467.]
{/net/bovine/home/kevin/ps/the/ch3/crid.eps}
  \caption{The SLD \v Cerenkov Ring Imaging Detector (CRID).}
  \label{fig:crid}
\end{figure}
 
 
The CRID is shown schematically in Figure~\ref{fig:crid}.
The barrel CRID sits between the CDC and the Liquid Argon 
Calorimeter (LAC).  In the
case of the endcap, the CRID is sandwiched between layers
of the endcap drift chamber.
 
Particles emit \v Cerenkov radiation in both a liquid and
gas radiator.  The photons are converted by photo-ionization
in gas.  The photo-electrons then drift to the end of the
detector where they are measured by proportional wires.  The
drift time yields information regarding the conversion depth.
From this information, photon rings may be reconstructed and
the \v Cerenkov angle measured.
 
 
\subsection{Calorimetry}
 
The SLD calorimetry is comprised of three complementary
systems.  The bulk of the electromagnetic and 
hadronic energy measurement is performed with the 
lead-liquid argon calorimeter (LAC).  The warm iron
calorimeter (WIC) provides measurement of the remainder
of hadronic showers.  The Luminosity Monitor and 
Medium Angle Silicon Calorimeter complete the
electromagnetic coverage at small angles.
 
\subsubsection{Liquid Argon Calorimeter}
 
 
The SLD LAC design is
based on a new understanding of the compensation
mechanism for sampling calorimeters \cite{lac,jebtg}.  
For liquid
argon devices, equalization of electromagnetic and
hadronic response can be achieved by using a high Z
absorber such as lead or uranium.  This leads to
an overall improvement in hadronic energy resolution
due to the minimization of fluctuations of the 
electromagnetic components of the shower.
 
Liquid argon is advantageous in that it allows for
arbitrary transverse segmentation, can be designed
for minimal dead regions, can reside inside the 0.6 T
magnetic field of the SLD solenoid, is radiation hard and
a unity gain
sampling medium.
Since the argon is a dense sampling medium, it can be
used with thin absorber layers to achieve excellent
electromagnetic energy resolution. 
 
 
As with other subsystems, the LAC is divided into a barrel
detector and two endcap detectors.  The barrel LAC 
(shown schematically in Figure~\ref{fig:blacass}) covers
roughly 80\% of the solid angle.  The aluminum dewar 
structure is suspended inside the SLD solenoid.  The dewar
contains one large liquid argon volume in which all of the 
lead plates and tiles reside.   The full azimuth of the
cylinder is spanned by 48 modules of width $\sim\!30$ cm.
Along the z axis, three modules comprise the length of
the spool, two end modules and one center module. 
 
 
\begin{figure}[hbt]
%  \hspace*{.1in}
  \epsfxsize=6.in
  \epsffile[-83. 115. 441. 490.]
{/net/bovine/home/kevin/ps/the/ch3/lacexp.eps}
  \caption[Barrel LAC Assembly.]{The barrel LAC
assembly.  The calorimeter is suspended inside the
SLD magnet.}
  \label{fig:blacass}
\end{figure}
 
 
The endcap LAC covers the angular region between 
8$^\circ <\theta < 35^\circ$ and is mounted on the
endcap warm iron structure.  An exploded view of the
endcap assembly is shown in Figure~\ref{fig:elacass}.
Like the barrel, the dewar
contains a single argon volume.  Inside the dewar, 16 wedge
shaped modules surround the beam axis.  
 
 
\begin{figure}[hbt]
%  \hspace*{.1in}
  \epsfxsize=6.in
  \epsffile
{/net/bovine/home/kevin/ps/the/ch3/ecass.eps}
  \caption[Endcap LAC Assembly.]{The endcap LAC
assembly.  The calorimeter is attached to the endcap
iron door structure.}
  \label{fig:elacass}
\end{figure}
 
 
The structure of the endcap and barrel modules is very similar.  
Figure~\ref{fig:bmod} shows a LAC barrel electromagnetic
and hadronic module.  A single endcap module contains
both the electromagnetic and hadronic section 
(see Figure~\ref{fig:emod}).
Both barrel and endcap modules are comprised of
an alternating array of lead plates
and tiles.  Figure~\ref{fig:laccell} shows the plate and tile
structure.  The plates are held at ground; the tiles are
held at negative high voltage and serve as the charge 
collecting electrodes.  Structure and electrical isolation
is maintained with a series of plastic spacers. 
The tiles are projective and joined
longitudinally to form towers.  Charge deposited in the
liquid argon is collected on the tiles which is then passed
outside the cryostat to the electronics.
 
 
\begin{figure}[hbt]
  \hspace*{.1in}
  \epsfysize=4.9in
  \epsffile[149. 261. 393. 486.]
{/net/bovine/home/kevin/ps/the/ch3/laccell.eps}
  \caption[A LAC Cell.]{One cell from the barrel LAC.}
  \label{fig:laccell}
\end{figure}
 
 
The lead plates and tiles in the electromagnetic (EM) section
are $2.00\, \mm$ thick and are separated by $2.75\,\mm$ liquid argon
gaps.  The first 16 layers (EM1) are readout in parallel, as are
the last 40 layers (EM2).  The EM1 section is roughly 6 radiation
lengths of material and the EM2 section is 15 radiation lengths
deep.  The EM section contains 98-99\% of
a 50 GeV electromagnetic shower. 
 
The hadronic section resides directly behind electromagnetic
calorimeter.  For the hadronic sections, the $2.75\,\mm$ argon
gap thickness is maintained, but the lead plate and tile thickness
is increased to $6.00\,\mm$.  The hadronic calorimeter is also
divided into a front and back section (HAD1 and HAD2).  The 
total thickness of the LAC (EM + HAD) is 2.8 nuclear
absorption lengths and contains 80-90\% of the hadronic
shower energy.
 
\begin{figure}[hbt]
  \hspace*{.1in}
  \epsfxsize=5.5in
  \epsffile[-100. -170. 548. 389.]
{/net/bovine/home/kevin/ps/the/ch3/lacmod.eps}
  \caption[A LAC Barrel Module.]{A LAC barrel module.}
  \label{fig:bmod}
\end{figure}
 
The barrel and endcap tower structure is projective.
In the barrel, 192 EM towers span the azimuth, each
with an opening angle of $\delta\phi = 33 $mr.  The
polar angle of the barrel is divided into 34 EM towers
on each side of the midpoint.  Azimuthal coverage in
the endcap is 192 towers at large angles, falling to
96 and then 48 at angles nearer the beamline.  The
polar angle is divided into 17 EM segments in the endcap.
The total tower count is 32448 for the barrel and 8640 for
the endcaps.
 
The design electromagnetic energy resolution is 
$10\!-\!12\%/\sqrt{E}$  
and is dominated by the presence of the aluminum
dewar in front of the calorimeter and shower fluctuations.
The resolution is worse than this near module gaps due to
charge collection inefficiency.  Also 
the resolution is poor in the endcap region, where additional material in
front of the calorimeter from the inner detectors and
associated electronics causes preshowering.
 
 
\begin{figure}[hbt]
  \hspace*{.1in}
  \epsfxsize=5.5in
  \epsffile
{/net/bovine/home/kevin/ps/the/ch3/ecmod.eps}
  \caption[A LAC Endcap Module.]{A LAC endcap module.}
  \label{fig:emod}
\end{figure}
 
\subsubsection{Data Acquisition and Trigger}
\label{se:lacacq} 
 
Signals are bussed out of the aluminum argon
vessel and into ``tophats'', which perform the
tasks of amplification, digitization and data
multiplexing.  Electronics and data acquisition
for the LAC is almost identical to that of the
Luminosity Monitor.  The data acquisition and trigger
will be
briefly outlined here and  discussed in 
detail in Chapter~\ref{ch:sldlmsat}.  Signals are
sent from the tophats via optical fiber to 
custom Fastbus Calorimeter Data Modules (CDMs).
 
The CDMs perform the trigger calculations at $120\,$Hz.
If a trigger condition is satisfied, the data from
the CDMs is sent to the AEB and eventually written
to tape.  The processors within the CDMs perform
several trigger tasks at 120 Hz in addition to 
performing baseline subtraction and application
of the proper calibration constants.
 
\subsubsection{Luminosity Monitor}
 
Electromagnetic calorimetry at small angles is
performed with two silicon sampling calorimeters:
the Luminosity Monitor and Small Angle Tagger (LMSAT)
and the Medium Angle Silicon Calorimeter (MASC).
The compact size of these detectors make them ideal
for the tight space constraints of the beamline region.
In addition, the fine transverse segmentation which
silicon allows makes it ideal for luminosity monitoring
by means of the \bha\  process.
 
The LMSAT will be discussed in detail in 
Chapter~\ref{ch:sldlmsat}.
 
\subsubsection{Warm Iron Calorimeter}
 
The WIC is a steel-limited streamer tube sampling
calorimeter and muon tracker \cite{wic}.  It also acts
as a magnet flux return, as well as an octagonal 
superstructure which supports the
remainder of the SLD.
The total thickness of the WIC is 4.2 nuclear
interaction lengths ($\lambda$), comprised of
14 steel plates 5 cm thick.
In addition, there are two iron endcaps
which support the remainder of the endcap 
components.
 
Wire planes between the steel layers are plastic
streamer tubes bundled together to form planar
chambers.  The WIC tower segmentation follows that
of the LAC. 
 
Muon tracking is also performed with the WIC.
The chambers contain copper strips which are
1 cm wide and run the length of the chamber.
These are read out digitally.  Muon tracks
are identified by matching extrapolated CDC
tracks with hits in the WIC strips.
 
\subsection{The WISRD}
\label{se:wisrd}
 
The energy of both the electron and positron beams
are measured on a pulse by pulse basis by the
Wire Imaging Synchrotron Detector (WISRD) \cite{nash}.  One
detector sits in each extraction line and is able
to measure the beam energy by the profile of
synchrotron radiation the beam emits as it is passed
through an analyzing magnet.  This is shown in 
Figure~\ref{fig:wisrd}.  
 
\begin{figure}[hbt]
  \hspace*{1.25in}
  \epsfysize=2.2in
  \epsffile[28.8 180 316.8 338.4]
{/net/bovine/home/kevin/ps/the/ch3/energy_s.ps}
  \caption[The Energy Spectrometer.]{The Wire Imaging
Synchrotron Detector (WISRD) Energy Spectrometer.}
  \label{fig:wisrd}
\end{figure}
 
 
 
 
\section{Polarimetry}
\label{se:compton}
 
In all electroweak measurements involving a polarized
electron beam, the magnitude of the polarization enters
linearly into the measured quantity.  Therefore, it
is crucial to measure and monitor the magnitude of the
electron beam polarization as accurately as possible.
 
The primary electron beam polarization measurements are
undertaken with the Compton Polarimeter \cite{comprop}.  This device
makes use of the cross section asymmetry in electron-photon
scattering.   The Compton scattering process is shown in Figure~\ref{fig:compton}.
Circularly polarized photons are backscattered off of the
electrons in the center
of momentum frame.  The scattered electrons are momentum
analyzed by means of a bending magnet and then detected
in both a \v Cerenkov detector and proportional tube detector.
By measuring the Compton asymmetry and the photon polarization,
the electron polarization can be extracted.
 
 
% compton physics
\begin{figure}[hbt]
  \hspace*{1.1in}
  \epsfysize=2.8in
%  \epsffile[7.2 72 583.2 748.8]
   \epsffile[108. 389. 350. 553.]
{/net/bovine/home/kevin/ps/the/ch3/compton_scat.ps}
%{/net/bovine/home/kevin/ps/pix/slc/compton_3.ps}
  \caption{The Compton Scattering Process.}
  \label{fig:compton}
\end{figure}
 
The layout of the Compton polarimeter is shown in
Figure~\ref{fig:compt}.  The electrons are brought
into collision with the laser approximately 40 meters
after the \ep collision point.  The laser fires at a
rate of 10.9 Hz, coinciding with every eleventh SLC pulse.
Data from every laser-beam crossing are used to measure the
electron beam polarization.
A statistical error of 1-2\% is achieved in a three minute
run.  The Compton data is shipped to the SLD data acquisition
system and written to tape asynchronously with the triggered
data from the other subsystems.
 
It should be noted that the helicity of each pulse is
transmitted by means of redundant lines from the Polarized Electron
Source directly to both the
SLD acquisition system and the Compton polarimeter on a
pulse-by-pulse basis.  The magnitude of the electron
beam polarization is measured every few minutes \cite{compton}.
The beam polarization assigned to each event is a combination
of the helicity from that beam crossing along with the polarization
magnitude from the nearest polarimeter measurement in time.
 
\begin{figure}[hbt]
  \hspace*{1.65in}
%  \epsfysize=1.7in
   \epsfysize=6.5truein
  \epsffile[605.5 607.5 1100.5 1192.5]
{/net/bovine/home/kevin/ps/the/ch3/compton_det.ps}
  \caption[Compton Polarimeter Layout.]{The Compton
Polarimeter.  Polarized photons are scattered off the
extracted electrons.  The electrons are momentum analyzed
to determine the beam polarization.}
  \label{fig:compt}
\end{figure}
 
 
In addition to the primary Compton polarimeter, two other
polarimeters utilize Moller scattering (\mol) to 
measure beam polarization. 
Both detectors make use of a foil target in an external
magnetic
field.  The polarized beam is then incident upon
a target of polarized electrons.  The Moller asymmetry
is small and more difficult to extract than is the
Compton asymmetry.
The Linac Moller polarimeter
is stationed at the end of the linear accelerator.  
Beam must be
diverted from the arc to the Linac Moller target.  The
Extraction Line Moller has only been used sparingly
due
to high backgrounds in the extraction line area.   Also, 
the location of the Extraction Line Moller is
incompatible with the normal operation of the WISRD.  
 
 
% this is the cerenkov
%\begin{figure}[hbt]
%  \vspace*{-0.4in}
%  \hspace*{0.9in}
%  \epsfysize=4in
%  \epsffile[151.2 230.4 345.6 385.2]
%\epsffile[151.2 230.4 100. 385.2]
%{/net/bovine/home/kevin/ps/pix/slc/compton_2.ps}
%  \epsffile[19 50.4 443.6 679]
%{/net/bovine/home/kevin/ps/the/ch3/compton_layout.ps}
%  \caption{QED annihilation process.}
%  \label{compton2}
%\end{figure}
 
% compton asymmetry
%\begin{figure}[hbt]
% \hspace*{1.5in}
%  \epsfysize=3in
%  \epsffile[-14.4 158.4 475.2 590.4]
%{/net/bovine/home/kevin/ps/the/ch3/compton_asym.ps}
%  \caption{compton4}
%  \label{fig:feynman_ann_ffbar}
%\end{figure}
 
% linac moller layout
%\begin{figure}[hbt]
%  \hspace*{0.75in}
%  \epsfysize=5in
%  \epsffile[-21.6 158.4 568.8 756]
%{/net/bovine/home/kevin/ps/the/ch3/linac_moller.ps}
%  \caption{linacmo}
%  \label{fig:feynman_ann_ffbar}
%\end{figure}
 
%
%\begin{figure}[hbt]
%  \hspace*{1.1in}
%  \epsfysize=5.5in
%  \epsffile[-14.4 -14.4 525.6 705.6]
%{/net/bovine/home/kevin/ps/pix/slc/moller_result.ps}
%  \caption{moller result}
%  \label{fig:feynman_ann_ffbar}
%\end{figure}
 
%
%\begin{figure}[hbt]
%  \hspace*{1.6in}
%  \epsfysize=2.8in
%  \epsffile[-7.2 151.2 439.2 561.6]
%{/net/bovine/home/kevin/ps/pix/slc/alcth_pol.ps}
%  \caption{polresults}
% \label{fig:feynman_ann_ffbar}
%\end{figure}
 
